1. Field of the Invention
This invention relates to iron-base alloys in general and, more particularly, to a corrosion resistant alloy which can be useful for flexible products, such as automotive exhaust components.
2. Description of Related Art
Operating requirements for automotive flexible exhaust couplings are becoming increasingly severe. Higher operating temperatures and more stringent emission requirements, along with extended warranties and government demands for increased gas mileage, are rendering conventional coupling alloys marginally acceptable or, more often, unacceptable for a growing number of engine platforms. Requirements for longer life demand corresponding improvements in fatigue and corrosion resistance properties of alloys.
In an automotive exhaust system, a bellows assembly is inserted between the exhaust manifold and the exhaust pipe. Due to the exacting requirements of modern catalytic exhaust systems, the bellows must permit the flexible routing of exhaust system components while simultaneously preventing oxygen ingress to the oxygen sensor.
Currently, bellows are comprised of a welded two- or three-ply metal tubular sheet which is partially corrugated to form a flexible bellows arrangement. Two- and three-ply designs typically utilize stainless steel (321 or 316Ti) inner layers. The outer ply can be made from INCONEL® 625 alloy or INCOLOY® 864 alloy. INCONEL® 625 and INCOLOY® 864 Ni—Cr alloys are commercially available from Special Metals Corporation of Huntington, W. Va. The thickness of each of the plys can range from about 0.006 inches (0.15 mm) to about 0.01 inches (0.25 mm). In some designs, the bellows are protected by an inner and outer mesh covering of stainless steel (304) wire braid.
The road salt applied for deicing purposes eventually degrades the bellows. Analysis has shown that the stainless steel bellows corrode due to hot salt corrosion and chloride stress corrosion cracking. In some applications in which the bellows is located close to the exhaust manifold, high temperature fatigue is a concern. The requisite flexible nature of the bellows ultimately leads to the corrosive- or fatigue-induced demise of the stainless steel. For this reason, manufacturers have been specifying INCONEL® 625 or INCOLOY® 864 alloys as the protective outer ply since it resists salt corrosion and fatigue.
Due to the competitive nature of the automotive industry, there is a demand for a flexible alloy that is cost effective, superior in corrosion resistance to stainless steel, and fatigue resistant. In other automotive applications, such as diesel exhaust gas coolers, good grain size control during high temperature brazing operations and good post braze fatigue properties are desired.